This invention relates to electrical resistance heating elements, and more particularly to formable thermoplastic laminate heated element assemblies.
Electric resistance heating elements composed of polymeric materials are quickly developing as a substitute for conventional metal sheathed heating elements, such as those containing a Nixe2x80x94Cr coil disposed axially through a U-shaped tubular metal sheath. Good examples of polymeric heating elements include those disclosed in Eckman, et al., U.S. Pat. No. 5,586,214 issued Dec. 17, 1996 and Lock, et al., U.S. Pat. No. 5,521,357 issued May 28, 1996.
Eckman et al. ""214 discloses a polymer encapsulated resistance heating element including a resistance heating member encapsulated within an integral layer of an electrically-insulating, thermally-conductive polymeric material. The disclosed heating elements are capable of generating at least about 1,000 watts for heating fluids such as water and gas.
Lock, et al. ""357 discloses a heater apparatus including a resistive film formed on a substrate. The first and second electrodes are coupled to conductive leads which are electrically connected to the resistive film. The heater also includes an over molded body made of an insulating material, such as a plastic. Lock, et al. ""357 further discloses that its resistive film may be applied to a substrate, such as a printed circuit board material.
Laminated heaters are also disclosed in Logan, et al., U.S. Pat. No. 2,710,909, issued Jun. 14, 1955 and Stinger, U.S. Pat. No. 3,878,362, issued Apr. 15, 1975. These laminated structures include partially cured rubber-like substances, backed with layers of glass cloth, such as disclosed in Logan, et al. ""909, or the use of a discontinuous layer of electrically conductive elastomeric material containing conductive carbon adhered to a pair of spaced-apart conductor wires bonded to a durable plastic material, such as Stinger""s polyethylene terephthalate film.
Other laminated heaters are disclosed in U.S. Pat. No. 2,889,439 to Musgrave, issued Jul. 29, 1955, and U.S. Pat. No. 3,268,846 to Morey, issued Aug. 23, 1966. Musgrave discloses a laminated heating panel including a resistance wire laminated between two sheets of asbestos paper impregnated with a phenolic resin or plastic. Morey discloses a flexible tape heating element and method of manufacturing the same. A resistance ribbon is sandwiched between a film of teflon, silicon rubber, or plastic material. There still remains a need, however, for a reformable but robust electrical resistance heated element which is easily adaptable to a variety of end uses and manufacturing processes. There also remains a need for a resistance heating element which is capable of capturing intricate circuit paths and which is reformable to provide efficient heating in complex heat planes.
The present invention comprises a heated element assembly and method of manufacturing heated element assemblies. A heated element assembly includes a first thermoplastic sheet, a second thermoplastic sheet, and a resistance heating element disposed between the first and second thermoplastic sheets. The resistance heating element comprises a supporting substrate having a first surface thereon and an electrical resistance heating material fastened to the supporting substrate, where the electrical resistance heating material forms a predetermined circuit path having a pair of terminal end portions. The resistance heating element also includes a first flap portion capable of rotation about a first axis of rotation where the circuit path continues onto at least a portion of the flap portion. The thermoplastic sheets and resistance heating element are laminated together to form a reformable continuous element structure. The continuous element structure is formed into a final element assembly configuration whereby at least the first flap portion is rotated about the first axis to provide resistance heating in at least two planes.
The present invention as described above provides several benefits. An intricate resistance circuit path of a resistance heating element may be secured to a planar supporting substrate and then laminated between thermoplastic sheets, whereby the planar resistance heating element may then be reformed with the laminated structure to provide heat on a plurality of heat planes. The heated element assembly may also be secured to a second heated element assembly to form, for example, a heated containment bag or a heated container. These heated structures provide intimate contact between the contents of the heated structures and the heat source, thereby providing inherent energy consumption advantages as well as the ability to intimately locate secondary devices such as thermistors, sensors, thermocouples, etc . . . , in proximity to the contents being heated or conditions being observed or recorded.
The heated element assembly also allows for an infinite number of circuit path shapes, allowing the circuit path to correspond to the general shape of a desired end product utilizing the heated element assembly. The heated element assembly may be folded to occupy a predefined space in an end product and to provide heat in more than one plane, thermoformed into a desired three dimensional heated plane, or stamped or die cut into a predetermined flat shape which may, then, be folded or thermoformed into a desired three dimensional heated shape. The heated element assembly thereby emulates well known sheet metal processing or known plastic forming processes and techniques.
The heated element assembly according to the present invention may also be over molded in a molding process whereby the resistance heating element is energized to soften the thermoplastic sheets and the heated element assembly is over molded with a thermoplastic to form a detailed molded structure. The energizing and overmolding steps may be timed such that the thermoplastic sheets and over molded thermoplastic form a substantially homogenous structure accurately capturing and positioning the resistance heating element within the structure. Alternatively, the heated element assembly may soften during mold flow without additional energizing.
In another embodiment of the present invention, a sheet of heated element assemblies comprises a first thermoplastic sheet, a second thermoplastic sheet affixed to the first thermoplastic sheet, and a sheet of resistance heating elements secured between and to the first and second thermoplastic sheets. The sheet of resistance heating elements includes a supporting substrate having a first surface thereon and a plurality of spaced circuit paths, each of the spaced circuit paths comprising an electrical resistance heating material fastened to the supporting substrate to form a predetermined circuit path having a pair of terminal end portions. Each of the circuit paths continue onto a first flap portion of a resistance heating element capable of rotation about a first axis of rotation. The thermoplastic sheets are laminated together such that the sheet of resistance heating elements is secured between and to the first and second thermoplastic sheets to form a reformable continuous element structure.
The sheet of heated element assemblies provides several benefits. The sheet may be inexpensively and efficiently produced using mass production techniques. The sheet may be collected into a roll, allowing the later separation and use of individual heated element assemblies or group of heated element assemblies as described above. The sheet, rather than being collected into a roll, may be further processed using various secondary fabrication techniques, such as stamping, die cutting, or overmolding.